Calculate the cash gap
Find the difference between the old and new annual commuting costs: fares or fuel, parking, tolls and office-day extras. Only count costs that genuinely change between the jobs.
Calculate the time gap
Compare annual commute hours. A move from 25 minutes each way once a week to 60 minutes each way four times a week can add hundreds of hours per year.
Gross up the difference
The new salary has to cover those extra costs after Income Tax, National Insurance and potentially student loan repayments. That is why a £4,000 effective-value gap may need a much larger gross salary increase.
Do not ignore pension terms
If the new employer contributes less to your pension, that is another part of the package you are giving up. Conversely, a strong employer pension can reduce the salary premium you need.
Negotiate around the break-even point
Your calculated break-even salary is a useful floor, not necessarily your target. A career move normally needs some upside for risk, probation, disruption and future uncertainty.